The "climate change is a myth" crowd is still at it, but their claims are growing simultaneously more shrill and less credible. The evidence is piling up at a
rate which is accelerating as fast as the atmospheric GHG concentration: glaciers
the world over are retreating, mountain snowpacks shrinking, New Mexico's piñon
pine forests dying en masse, permafrost across the arctic melting, the whole of
Antarctica losing ice mass while the West Antarctic Ice Sheet appears to be
destabilizing, Greenland's glaciers accelerated to double their previous speed,
the North Atlantic Conveyor slowing worringly, the Larsen B ice shelf broken
up, arctic sea ice vanishing, heat waves killing tens of thousands even in the first
world... the list goes on and on[1].

Some authors who appear to take the issue seriously nevertheless claim that
we can
wait half a century (p. 217) to act decisively about this. Once I would have
believed that we had lead time and could get mitigation measures into place before the
real crisis hit. I can't agree with this any more. We are already seeing
some GW effects occurring decades ahead of naïve predictions, and other effects
are likely to increase warming even further. Methane from thawing
permafrost across Siberia and Canada and CO2 from fires in drought-stricken
tropical rain forests are just two of the likely natural additions to human
GHG emissions.

We don't have the luxury of
either complacency or time; failure to act will allow the existing positive feedbacks
to accelerate beyond anything we could to do arrest them. We need to take this
seriously and do something to halt the progressive warming and its effects, NOW. We
may wind up with an environment falling to pieces if we don't do something.

What do you want me to do?

Some people react to the idea with a shrug, saying "What can I do about
it?" If there's no remedy, there's no point in getting all worked up about
it, is there? It's like growing old: something to be handled when it comes,
planned for, but not worth worrying about at the cost of today. This seems to be
the attitude pushed by those who have an interest in denying global warming but who have
been forced to concede to the evidence. Some just go back to denial as the
most comfortable response. But what if there was something we could do?

That something would have to meet several requirements:

It would have to be something that could be put into action quickly.

It would have to be inexpensive, so a small government appropriation or even a large private effort could take it through the pilot stages.

It would have to be effective on the scale of one nation's effort, to avoid years of
inter-government negotiations.

If something could be done, quickly and cheaply... it would change everything. Some
might oppose it as a license to continue burning oil and switch more demand to coal,
but the psychological impact would be immense. Yes, we are having large
effects upon our climate. Yes, we do have the capability - and thus the
responsibility - to do something about it. It would be a step in the right direction.

If it's broken, don't break it worse

The difficulty with trying to fix a problem on this scale is that the
solutions must not have side effects anywhere near as large. Such
things get into a nasty mess of fixes, counter-fixes, counter-counter fixes
and so forth. Ideally the fix will work just like some natural
mechanism that already does just what we need. It needs to be something
that we can apply easily, and adjust or stop entirely if it has untoward
side effects.

Fortunately, nature gave us an existence proof: aerosols.

The 1991 eruption of Mt. Pinatubo provides an example that we can
follow. Up to 20 million tons of material was injected high into the
atmosphere (my sources are not clear how much of that got to, and remained
in, the stratosphere). The sulfur which got there oxidized to SO3 and
then combined with water to form H2SO4; this material remained
aloft for as much as 3 years.

The effects included:

Ozone depletion (catalysis on the surfaces of the aerosol particles)

Temperature decreases up to 0.6° C in the northern hemisphere

A somewhat lesser temperature decrease in the southern hemisphere

A drop in sea levels of approximately 0.5 centimeter

Except for the ozone depletion, this is just what the doctor ordered. It
is effective, it is reversible, and the world certainly did not end as a
consequence. Far from it; some research concludes that soil carbon inventories
increased as a consequence of the atmospheric effects. This not only helped
offset the effect of warming, it directly affected the cause.

But can we do what Pinatubo did, and keep it up year after year while we
deal with the root cause? Let's take a look at that.

Emulating nature

Pinatubo injected on the order of 20 million tons of sulfuric acid into
the mid-stratosphere, to 100,000 feet or so. Volcanic emissions contain
a great deal of steam which condenses almost immediately, so a substantial amount
of this probably fell out in a relatively short time; however, some of the
clouds reached upwards of 120,000 feet and remained aloft for years. Could
engineering duplicate this feat of nature?

I think so. And not just the same, but better:

The sulfur could be placed at the best altitude for the desired effects.

The sulfur could be injected at the best geographic locations ditto.

The sulfur could be added in the optimal or most convenient chemical form (SO, SO2, SO3, H2S).

Other details could also be chosen to suit.

Completely offsetting the anthropogenic greenhouse-induced warming requires
reflecting roughly 1% of Earth's incident sunlight. This sounds like a big
job, but coal-fired powerplants in the USA alone handle far more than 20 million
tons of sulfur per year. This sulfur mostly winds up in scrubbers and on
relatively nearby land (fallen out or rained out), but if it could be captured
in a suitable form we could certainly put it elsewhere. If the lifespan
of the sulfur in the stratosphere is on the order of years, it might take far
less than 20 million tons per year to slam the brakes on global warming.

Atmospheric lifespan is something I know next to nothing about (aside from
the sketchy information I have about the duration of Pinatubo's effects), but
I was able to find something about the scattering efficiency of sulfate aerosol
particles. According to
figure
1 of this abstract [2],
the effective surface area of sulfate particles even under the least effective (driest)
conditions has a broad peak around 10 square meters per gram for optimal particle
sizes. The Earth has a total surface area of approximately 5.12*1014
m2, so intercepting 1% of the sunlight needs a reflector of 5.12*1012
m2. At an effective surface area of 10 m2/g, this could
be accomplished with as little as 512,000 metric tons of sulfate in the atmosphere at any
time. Unfortunately, most scattering occurs at relatively small angles so
the total amount of material required to reflect that much light away from Earth
is several times as much.

The abstract also gives a graph of radiative forcing vs. dry particle size and
relative humidity. According to Figure 2, the peak forcing for 40% RH comes
to about 250 watts per gram. A 1% reduction of insolation is a forcing of
roughly 1.74*1015 watts; at 250 W/g, it would require around 7 million
metric tons of aerosol particles. Greater levels of relative humidity could
cut the required amount by almost a factor of eight.

On the scale of human effort, this is nothing. This is only a few times
the tonnage of airliners flying at many times of day, and they can only
remain aloft for hours; sulfate aerosols can last years. Ecological
impact would probably be minimal. If the sulfur was drawn from industrial
sources which are currently going into the troposphere, both the total amount
emitted and its chemical effect on the landscape could be slashed drastically.
Compared to the status quo, it looks like a great improvement.

The rate of fallout would average the same as the rate of injection. This
rate is broadly equivalent to the required mass of particles divided by the
lifespan. The lifespan would be partly determined by the exact location of
injection, the chemical form and the weather conditions in the stratosphere. I
have no information on these so I will not address them further.

Getting it

The USA is currently burning approximately 1 billion tons of coal per year, 90%
of which goes into electric powerplants. If this coal averages 3% sulfur, the
annual sulfur flow is 30 million tons (30 million tons of sulfur can make over
90 million tons of sulfate ion, SO42-). If the current
level of GHG's requires 7 million tons
of sulfate aerosols to offset them and the aerosols have a lifespan of 2 years,
the replacement rate is 3.5 million tons of sulfate per year. If we assume
100% conversion, this could be added as 3.6 million tons of H2SO4,
2.9 million tons of SO3, 2.3 million tons of SO2, or
1.24 million tons of H2S. This is on the order of 4% of the
sulfur in the coal mined in the US each year: clearly not a big difficulty
to obtain.

Where to get it seems obvious, but the question becomes how to get it in a usable
form? So long as we intend to burn coal to make electricity for the next decade
or two, the most viable option seems to be to convert powdered-coal combustion plants
to one or another type of
IGCC. Oxygen-blown
IGCC yields sulfur as H2S
when the fuel gas is scrubbed, and air-blown IGCC may produce it as ZnS which
is further oxydized to ZnO and SO2; design may permit capture of pure
SO2 in the scrubber effluent. Either gas could be liquefied
under pressure and stored until it could be dispersed.

The use of IGCC with cold-gas cleanup is desirable for other reasons as
well; it offers almost complete elimination of fly ash and other particulates,
sulfur can be scrubbed to levels well below EPA requirements, and both mercury
and other toxic metals can be scrubbed to 99% removal with activated-carbon
filtration of the cold fuel gas. On top of all that, the thermal efficiency
can be improved from 33% to 40% or greater. These improvements turn a
coal-burning plant into a far better neighbor.... almost a good one.

Putting it where it needs to go

Supposing that we've got a million tons or two of liquefied sulfur gases: how
do we get them to where they do the most good? This leaves the realm of
chemistry and takes us to aviation. One possibility is to mix some hydrogen
into the gas to make it sufficiently lighter than air, and then release balloons
filled with it; if the balloons burst (or were set alight) reliably at a known
altitude the dispersion could be controlled rather precisely. This classic
method would be immediately understood by the Mongolfier brothers.

If aerostats are too fragile, too unreliable or too toxic (who wants the
results of a burst balloon near ground level?), aerodynes are the obvious
step up. Modern jumbo-jet engines are powerful, efficient, and relatively
quiet; an ultra-short haul aircraft might be designed around one to fly freight
from the ground to 24 miles up, one way. Let's take the GP7000 engine as
our baseline. The GP7277 variant is 6033 kg of machinery with a static
thrust of 342.5 kilo-Newtons; it can (theoretically, under standard conditions)
lift almost 5.8 times its weight. If a drone was built around a GP7277 core
and its airframe, tankage, and other systems weighed as much as the engine, it
could lift as much as 22.8 metric tons of cargo on a vertical takeoff.
Bypass-burning could increase this figure considerably, at the cost of new
enclosures of a heat-resistant material like Inconel or titanium.

If such a drone could take 20 metric tons of gas to altitude on each flight,
the system would require roughly 62,000 flights per year carrying H2S or
117,000 flights per year carrying SO2. A fleet of 100 drones
could handle this at less than 2 flights per day carrying H2S, and
less than 4 flights per day carrying SO2. Many commuter aircraft
fly more legs each day over longer distances, albeit not to such extreme heights.

If aircraft won't do the job, we could always bring in the big guns... literally.
Firing tanks of gas to the desired altitude and dumping them might be the most
efficient and cheapest method of all. Tanks could be recovered by parachute.
I'd worry about noise and misfires, though.

Could we capture a million tons or so of sulfur each year? Absolutely; at
about 8.5 tons per thousand megawatt-hours, the conversion of only 17 GW of coal-fired
generators could supply everything we need. Could we build a fleet of 100 drone
aircraft around jumbo-jet engines? If Scaled Composites can make a flying model of
an orbital spacecraft and power it with 4 RL-10 motors, I'm sure we could. Could
we have something next year? Hand a signed check to the right people and then stay
out of their way, and I bet it would happen.

We appear to have the way, and then some; all we need now is the will.

Footnotes:

My first reaction is, isn't sulfuric acid really nasty stuff? But perhaps at the low low concentrations falling out, it wouldn't do much to anything?

My second thought is, how do we get anyone to spend money on this? There's enough FUD out there about GW as it is; can you imagine trying to quantify with certainty the success/failure of the sulphur injection program for investors?

Third, how do we get around the ozone depletion? I suppose it's a secondary concern to slowing global warming, since the ozone layer will recover eventually. But, if there's a way around it, that would be even better.

Fourth, since the solution to global warming will probably require widespread use of solar power, should we also think about not going the other way and causing global cooling? (somewhat off topic, I know...)

Sulfuric acid IS nasty stuff, but we're talking a lot less spread over the whole globe than is currently emitted in the Ohio valley alone (and mostly dumped on the Appalachians). If we get it by putting some fraction of that sulfur in the stratosphere and appropriately disposing of the rest, the net benefits will be substantial.

Somebody like Bill Gates might finance a demonstration project. There are several very wealthy individuals who could easily put together the necessary money and talent to give the idea a short test. Scientists would love to have a controlled experiment to watch.

The issue of ozone depletion is a sticky one, and I don't think we can get around it. On the other hand, fine aerosols scatter blue light much better than red, and ultraviolet best of all. We might see less UV at the ground despite less ozone, and the effect would be gone in a couple of years after halting the injections anyway.

I do think there are people who would want to continue business as usual after GW was "solved", but the whole business of engineering the climate to avoid the consequences of our own emissions presents a strong argument for going the other way. Besides, if we're getting the sulfur from coal we can't leave ourselves with 600 ppm of CO2 in the atmosphere when we run out of coal, can we?

Another thing I wonder is how stratospheric particulates affect the rate of destruction of CFC's. If those droplets accelerate their conversion to hydrochloric acid, we might have something which solves as much of the ozone-depletion problem as it causes.

I think something like this would work, but would it have to be sulfur? Perhaps there are more efficient or benign chemistries. Maybe there are some large areas of dark earth that could be cheaply covered with white stuff, avoiding the need to put things in the atmosphere.

I would think that "damaging the ecology" would be the least of our worries if we are trying to halt a runaway warming. Why would it have to be effective in micron thickness? Why would it have to last years? As cheap as it would be to spread stuff on the ground, relative to launching it into the stratosphere, I'd think you could afford to redo it occasionaly. There's all kinds of light colored minerals. It's not like you'd need Analytical Reagent Grade, either.

"I would think that "damaging the ecology" would be the least of our worries if we are trying to halt a runaway warming."

I would agree. I especially think that using a "pollutant" that we are already dumping in far greater quantity but at a different altitude would be a net improvement over the status quo.

"Why would it have to be effective in micron thickness? Why would it have to last years?"

If you go from microns to millimeters, you've multiplied the required quantity of material by a factor of a thousand. If it only lasts months, your rate of application jumps. The energy requirements scale roughly as the total mass.

"As cheap as it would be to spread stuff on the ground, relative to launching it into the stratosphere, I'd think you could afford to redo it occasionaly."

If you need a thousand times as much material on the ground, you can pay a hundred times as much per pound to put it in the stratosphere and still have only 1/10 the outlay.

This is a great piece of creative thinking by Engineer-poet. I think we need more people coming up with possible solutions like this. I have no idea how this idea compares economically with say seeding the Southern ocean with iron or other solutions that have been suggested, but the more options we have the better, and there is no reason why several different approaches couldn't complement each other. Good work.

This isn't anything that others haven't come up with before, I just think it's the easiest and fastest stopgap we could use.

And it's a stopgap, not a solution. The real solutions are going to involve carbon sequestration, like seeding the oceans with iron (which I understand doesn't work as well in practice as in theory) or massively boosting soil carbon via some method such as terra preta. Some folks are already talking about using it to mitigate climate change.

I don't come up with most of this stuff, I just put the pieces together, add numbers and bake. If I get something that's better than half-baked, I call it a success.

"I don't come up with most of this stuff, I just put the pieces together, add numbers and bake." Yeah, but putting the numbers in is a pretty big deal. Without people like you I'd have no idea of what is practical or not. For example, when someone tells me we can zap greenhouse gases with lasers I have no choice but to ignore them unless they can show me some maths that at least makes some sort of sense.

Their attitude can be summed up as "I know an old woman, who swallowed a fly...". For instance, acidification of the oceans will proceed merrily along with increased CO2 levels (and maybe with a bit of acid rain to help it along.)

Acidification from the aerosols themselves could be less (the world emits tens of millions of tons of sulfur per year, this effort would take less than a tenth of that) and the CO2 would be less of a problem if more of it was dissolved in arctic waters which became deep ocean water. Carbonates aren't stable below a certain depth anyway, and if we can protect surface life from heat stress we'll be better off.

Then there's the effect of aerosols on carbon uptake. Apparently, Pinatubo's effect allowed plants to absorb more carbon than they would have otherwise (less water stress, less respiration); you can see the flattening of the Keeling curve in 1992 and 1993 here. If that's all it takes to give us breathing room to fix the problem before it fixes us, I'm all for it.

I just wonder one thing : how many balloon should we send in the stratosphere each year ? Crutzen, who won the nobel price of chemistry, say that we would have to send 1 million tonnes of sulfure each year. Do you think it is possible in balloons ??